4. THE GASEOUS X-RAY-EMITTING MEDIUM

There are a number of interesting questions we would like to answer
when studying the gaseous X-ray emitting environment around a radio
source. Is the external gas pressure greater than or equal to the
minimum pressure calculated for the jet, in which case alternative
methods of jet confinement are not required? We would like to know if
the radio galaxy is moving in the X-ray medium (so that the jet is
affected by ram-pressure forces), or if there are large-scale gas
motions (cooling-flows, mergers, winds, etc) which affect the
production of jets or cause their disruption. Is the gas distribution
smooth between large and small scales, or do abrupt transitions in
temperature and density induce observable radio deformations? Do we
see evidence of direct interaction between the jets and the
surrounding medium (e.g., heating of the X-ray gas) which can be
tested against model predictions?

We can compare the inferred age of a radio source with the timescale
over which the environment is likely to change.

The sound crossing time in gas
of size d is
2 (d / kpc)
(kT / keV)-1/2 Myr.
This means that a medium 100 kpc in size
has not had time to change as a result of the presence of
a 100 Myr-old radio source, and a young radio source
should be in an environment which is similar to that of
its older counterparts.

The cooling time of gas of
temperature kT and density ne is
3 ×
104 (kT / keV)1/2 (ne /
10-3 cm-3)-1 Myr. Wide ranges of
temperature and density relate to
a wide range of cooling times, in many cases approaching the Hubble
time. If a cooling-flow is key to the fuelling of a radio source, as
suggested for powerful radio galaxies by
Bremer et al. (1997),
then it is curious that most radio sources appear to be 100 Myr old or
younger, where we might expect some to last for a Gyr or more.

The phases of development of an
elliptical-galaxy atmosphere (supernova wind, density increase, cooling,
etc; e.g.
Ciotti et al. 1991)
are long compared with the measured
lifetimes of radio galaxies. Are the host galaxies of
radio sources in one of these phases, or all?

Hot atmospheres have been detected around FRI radio galaxies with
Einstein and ROSAT. For representative results I turn to the
largest sample of such objects with sensitive pointed X-ray
observations: the B2 radio-galaxy sample. This is a 408 MHz
flux-limited sample of 50 radio sources identified with elliptical
galaxies of mZw 15.4 mag
(Colla et al. 1975,
Ulrich 1989),
of which 40 were observed in ROSAT pointings, 39 being on-axis
(Canosa et
al. 1999).
Apart from one starburst galaxy and one BLRG, all are FRIs at z
0.072. Two of the galaxies are the dominant members of catalogued
Abell clusters (A1795 and A2199), and two are galaxies of the Coma
cluster. The environments of other sample members are measured
through their X-ray observations, with particularly useful data for
sources observed with the PSPC
(Worrall &
Birkinshaw 1999a).
Fig. 6 shows four representative X-ray images,
illustrating group to cluster scales typical of X-ray emitting
atmospheres. Such gas engulfs the radio structures
(Fig. 7), but the lack of correlation between
radio-source size, and size or central density of the X-ray-emitting
medium, means that the gas has indeed not had sufficient time to
adjust to the presence of the radio source
(Section 4.1), and
it must be small-scale processes, on size scales less than those of
the overall gaseous environments, which are the major influence on
radio-source dynamics and propagation.

Figure 7. The X-ray atmospheres (grey
scale) of the B2-sample galaxies
in Fig. 6 are substantially larger than
the VLA radio structures (contours). Note the change of scale from
Fig. 6. Radio data are at 5 GHz for NGC 2484 (image kindly provided by
M. Birkinshaw) and 1.4 GHz for the other sources (images kindly
provided by R. Morganti).

Although the ROSAT PSPC's spectral resolution is poor by the standards
of the CCD detectors on ASCA, Chandra, and XMM, and of Astro-E's
calorimeters, the energy band is well matched to the typical
temperatures of groups and poor clusters. The PSPC-derived
luminosities and temperatures of the environments of B2 radio galaxies
lie close to an extrapolation of the luminosity-temperature
(Lbol - kT) correlation for more-luminous
optically-selected clusters (Fig. 8). Since
Lbol is principally
governed by the gas mass, and kT by the total gravitating mass, this
implies that the presence of the radio galaxy does not affect the gas
fraction of the environment.

Figure 8. The X-ray-emitting atmospheres of
representative B2 radio galaxies with
good ROSAT PSPC measurements fit an extrapolation of the
luminosity-temperature (Lbol - kT) correlation for
more-luminous (~ 1044 - 1046 ergs s-1)
optically-selected clusters
(Arnaud &
Evrard 1999;
dotted lines show rms spread). This implies that the radio galaxy does
not greatly influence the gas fraction of the environment. Figure from
Worrall &
Birkinshaw (1999a).

The gas densities for the atmospheres of B2 radio galaxies do not
generally suggest the presence of cluster-scale cooling flows - the
exceptions being for the two Abell clusters, A2199 and A1795.
A2199 is a particularly interesting case, where
Owen & Eilek
(1998)
have pointed out that the rotation measure of the core of B2 1626+39
(3C 338) implies appreciable central magnetic energy
density, complicating the interpretation of any cooling flow.
Possible galaxy-scale cooling flows, which may play a role in fuelling
the radio galaxies, need further investigation using
the sensitivity and spatial resolution now available with Chandra.

Figure 9. Thermal pressures in the
atmospheres of six B2-sample radio
galaxies as deduced from fits to their ROSAT PSPC images (solid line,
shown dashed where extrapolated beyond region of clear X-ray
detection) compared with minimum internal pressure estimates
in the radio sources (horizontal bars). The intergalactic medium
is sufficient to confine
the outer parts of the radio structures, and in some cases even to
within 10 arcsec (5-10 kpc) of the core. In the case of NGC 315 the
(extrapolated) pressure of the atmosphere matches the minimum pressure
in the radio source over a factor of ~ 100 in linear scale.
Figure from
Worrall &
Birkinshaw (1999a).

Figure 10.NGC 6251. 330 MHz radio contours
on ROSAT PSPC image (left)
and X-ray radial profile with best-fit model of unresolved emission
plus weak group-scale gas described by a
-model (right).
Radio jet features between 10 arcsec and 4.4 arcmin from the core are
all overpressured with respect to the X-ray medium in this giant
radio source. Figure from
Birkinshaw &
Worrall (1993).

This review will not attempt a detailed discussion of how bending and
disruption of the kpc-scale jet structures of low-power radio galaxies
may relate to the motion of the radio galaxy through the gas or vice
versa. However, various factors are likely to be influential,
including gas flows and density enhancements resulting from cluster
mergers (e.g.
Bliton et al. 1998),
density and temperature
discontinuities at the interface between the galaxy and cluster
atmospheres (e.g.
Sakelliou &
Merrifield 1999),
and buoyancy forces (e.g.
Worrall et al. 1995).

A major success of ROSAT has been the first detection of high-power
radio galaxies at high redshift. Of the 38 radio galaxies at z >
0.6 in the 3CRR sample
(Laing et al. 1983),
12 were observed in ROSAT
pointed observations and 9 were detected (see summary in
Hardcastle
& Worrall 1999a),
with the four most significant detections exhibiting source extent
(Worrall et al. 1994,
Hardcastle et
al. 1998b,
Dickinson et
al. 1999).
Moreover, extended emission is
detected around five 3CRR quasars at redshifts greater than ~
0.4, one of which is at z > 0.6
(Hardcastle
& Worrall 1999a,
Crawford et
al. 1999).
Fig 11 plots the extended luminosities for
sources for which the structure can be well modelled, together with
upper limits for the other 3CRR FRII sources observed in ROSAT pointings
(roughly half the sample). Powerful radio sources are finding some of
the highest-redshift X-ray clusters known to date, pointing to deep
gravitational potential wells early in the Universe.

The nearer a source, the more likely it is that its various X-ray
emission components can be separated and the better will be the model
fitting to any extended emission. FRII sources are rarer than FRIs
and thus typically more distant. Cygnus A and high-redshift FRIIs
with good X-ray data have extended X-ray luminosities one to two
orders of magnitude higher than a typical FRI, but what about other
more local FRIIs? Their extended emission should be as easy to detect
if it really is so luminous. The situation appears mixed, with the
extended luminosities for 3C 98 (z = 0.0306, Lx
~ 1042 ergs s-1) and 3C 388 (z = 0.0908,
Lx ~ 1044 ergs s-1)
differing by two orders of magnitude, and atmospheres for many sources
not yet detected (Fig 11). This luminosity
range spanned by 3C 98 and 3C 388 is similar to that of representative
low-redshift
FRIs (see Fig 8), although the full distribution of
extended X-ray luminosities for FRIIs is uncertain while many
nondetections remain. Despite this, an interesting picture emerges.
Contrary to earlier work with less sensitive data
(Miller et al. 1985),
the X-ray atmospheres, where detected, provide sufficient pressure to
confine the radio lobes, with no disagreement from the many sources
for which only X-ray upper limits currently exist
(Hardcastle
& Worrall 1999c).
In a detailed study of 3C 388,
Leahy & Gizani
(1999)
have argued that that this implies the lobe
energy density is higher than given by minimum-energy arguments, and
they make the interesting point that if this is the case, jet
kinematic luminosities (normally calculated as energy density times
volume, divided by spectral age) are underestimated.

GHz Peaked Spectrum (GPS) radio sources are believed to be young FRII
sources and, even if only ~ 100 pc in size, the sound-crossing
time in the surrounding medium (~ 105 years:
Section 4.1) is likely to be appreciable compared
with the age of the source
(Conway 2000).
We therefore expect the environments of such
sources to be similar to those in the inner parts of their older
counterparts. A search with ROSAT and ASCA for X-ray emission in or
around the archetypal GPS radio galaxy 2352 + 495, at z = 0.237,
has set an upper limit for the soft X-ray band (0.2 - 2 keV) of about
2 × 1042 ergs s-1
(O'Dea et al. 1996,
O'Dea et al. 1999).
From Fig 11, this is already below the level at
which the atmospheres of some FRII radio galaxies are detected,
suggesting that slightly more sensitive observations with forthcoming
missions should see the atmosphere of this source.